The invention is in the field of x-ray tubes and particularly x-ray tubes used in industrial applications. Such tubes are available, for example, from X-Ray Technologies, Inc., Santa Cruz, Calif., and can be used in applications such as elemental analysis of materials by x-ray fluorescence. In such analysis a sample is excited with x-rays from a primary source and in response releases x-ray photons having energies characteristic of the elements present in the sample. The released photons are detected and their spectrum is displayed and/or used to deduce the elemental composition of the sample. Exemplary systems of this type are the XR 200/300 energy dispersive x-ray fluorescence spectrometers made by the U.K. company Link Analytical Limited and described in the company's brochure XR200/300/5/87/10M which is hereby incorporated in this disclosure by reference.
Typically, such x-ray tubes have a single x-ray target which is bombarded with electrons and generates x-rays leaving the tube housing through a side window or an end window. The x-ray characteristics are determined by factors such as the target material and geometry, the x-ray take-off angle (the angle of the utilized x-ray beam relative to the plane of the target), the nature and geometry of the electron source and the voltage difference between the electron source and the target. In addition, it is believed that some manufacturers have made x-ray tubes with several targets but a single electron source and either a single window or several windows, for x-rays which have respective take-off angles and different x-ray properties. Still in addition, it is believed that samples of x-ray tubes with twin, coplanar targets from different materials (iron and molybdenum), with a respective individually wired filament for each target but a single side window for the beams from the two targets, were sold in this country more than a year before the filing date of this application, for use in feasibility studies. Still in addition, it is believed that dual target tubes may have been sold in this country by Ortec, a company in Oak Ridge, Tenn., and that twin target x-ray tubes may have been discussed in a paper entitled " System Design Principles for Long X-Ray Tube Life for a New Dual Target End Window XRF Tube" presented as paper No. 633 by R. Barrett of The Machlett Laboratories, Inc. at The Pittsburgh Conference & Exposition On Analytical Chemistry and Applied Spectroscopy held Feb. 22-26, 1988 in New Orleans. La. Still in addition, it is believed that an x-ray tube was sold in this country and used to study jet exhausts more than a year before the filing date of this application, had a conical target of a single target material and generated x-rays in response to a cylindrical electron beam coaxial with the conical target, with the x-rays taken out through an annular window coplanar with a section of the cone normal to the cone axis. A twin or quadruple target x-ray tube using a single end window and individual filaments for the targets, is proposed in Machlett U.S. Pat. No. 4,017,757. No internal collimation separating the x-rays from the individual targets, and no use of separate windows, appear to be taught in that patent.
While these and similar prior art proposals can be useful in generating x-rays suitable for a desired purpose, it is believed that a need still remains for an x-ray tube which can generate purer and better controlled x-ray beams at higher efficiency, with reduced cross-talk between the different electron and x-ray beams, and with tube geometry allowing close coupling and convenient and effective collimation. This invention is directed to meeting such needs.
An x-ray tube in accordance with an example of the invention comprises a multifaceted anode with targets which are in different planes and are made of different materials. Each target has its individually controlled electron source, allowing the targets to be turned on and off in any desired pattern and/or sequence. The x-rays from the different targets are taken out at a high efficiency angle and through respective internal collimators and respective individual end windows. To suppress cross-talk between the different electron and x-ray beams and enhance the purity of the x-ray beams, the invented structure provides a particularly effective geometry of separate electron guns, non-coplanar and well separated targets, good control over electric fields, and separate internal collimators and windows and close coupling to the ultimate targets of the respective x-ray beams. Substantially the entire target can emit x-rays or, alternatively, one or more of the electron beams can be shaped such that their targets emit x-rays only or primarily from a focal spot which is smaller than the entire target area.
In a particular exemplary and nonlimiting embodiment of the invention, the x-ray tube has a target support which has a multifaceted, e.g., generally pyramidal front end. In order to reduce cross-talk through effective control over the electron beams and effective separation between the targets, the generally pyramidal front end has channels where its ridges would normally be and has a central axial bore instead of a forwardly projecting tip. Each facet has thereon an x-ray target which is made of a different material and therefore emits different x-rays when energized by its own electron gun. The target material may extend into the channels and the axial bore but the targets remain spaced from each other. In order to further reduce cross-talk and enhance the purity of the x-ray beams, the outer edges of the channels and the axial depression are rounded. The x-ray tube housing has a respective separate, end window for an x-ray beam from each of said targets. Internal collimation separates the x-ray beams from the respective targets to further reduce cross-talk and enhance beam purity. The respective x-ray beams can be parallel to each other and to the axis of the target holder or can be directed along respective different beam axes. The x-ray tube can be used in equipment having a network for individually controlling the electron guns to generate a selected pattern and/or sequence of said individual x-ray beams. When the x-ray tube is used for elemental analysis of materials, the equipment can select only the x-ray beam appropriate for the material being analyzed, or can sequentially use all or some subset of the available x-ray beams or can use a pattern of all or a subset of the beams or a sequence of patterns of subsets, all without changing the x-ray tube or the KV. Other desirable effects can be obtained by varying the tube KV and/or the cathode current as well.
An alternate embodiment also has a number of targets but they are substantially coplanar and are excited with electron beams which are substantially perpendicular to the plane of the targets. The targets emit x-rays in the directions of the electron beams. This embodiment is believed to be particularly useful as a replacement for radioactive sources which is easier to control and more versatile.